[U-Boot] 83xx, uec: adjust enet_interface settings on the fly.

[Liu Dave-R63238]

> > The code was assuming the initial speed is 1000Mbps, so the 
>> 1000 case did nothing.
> 
> Hmm.. but what happend, if you connect a 100Mbps, and then 
> back to the 1000Mpbs?

The ethernet port may be broken :)